1. Field of the Invention
The present invention relates to fuel electrodes for solid oxide fuel cells and their method of manufacture.
2. Background Information
Solid oxide electrolyte fuel cells ("SOFC") are well known in the art and are described generally in U.S. Pat. No. 4,490,444 (Isenberg). Such an SOFC can be of a flat plate, tubular, or other configuration, and typically operates at a temperature of from 700.degree. C. to 1100.degree. C., with a solid ceramic electrolyte disposed between two solid ceramic electrodes. In operation, one electrode is in contact with oxygen or air ("air electrode") and the other electrode is in contact with fuel ("fuel electrode").
Nickel/zirconia cermet (ceramic-metal) fuel electrodes are typically used for SOFC's, as exemplified by the tubular configuration of the Isenberg '444 patent. Seven properties, in particular, are desired to produce a commercially viable fuel electrode of this type. These are: high conductivity; adherence which does not deteriorate due to aging or thermal cycling; a large area of electrochemically active sites at the electrode/electrolyte interface; chemical and physical stability over a wide range of fuel atmospheres; minimal microstructural changes with operating time (since such changes are often accompanied by deterioration of SOFC voltage performance); adequate porosity, that is 25% to 45% porous by volume, to minimize concentration polarization; and, very importantly, lower fabrication costs.
A variety of procedures have been tried to satisfy these requirements, as described in U.S. Pat. Nos. 3,503,808 and 5,227,258 (Agruss et al. and Ito et al., respectively); Satoshi Ohara et al., Proc. Fifth International Symposium on Solid Oxide Fuel Cells, Vol. 97-40, p. 815; A. Tsoga et al., ibid, p. 823; R. Wilkenhoner et al., ibid, p. 851; Tomoo Iwata, J. Electrochem Soc., 143, p. 1521 (1966); an Nguyen Q. Minh, J. Am. Ceram. Soc., 76 [3], p. 563-588 (1993).
The most successful process to make these fuel electrodes to date has been the use of an electrochemical vapor deposition ("EVD") method. This EVD method forms a yttria stabilized zirconia "skeleton" within and surrounding a porous matrix of nickel particles, as described in U.S. Pat. Nos. 4,582,766 and 4,597,170 (Isenberg et al. and Isenberg, respectively). In one embodiment of this EVD process, nickel fibers, about 3 mm to 15 mm long and 0.01 mm to 0.05 mm diameter, are deposited within the fuel electrode as described in U.S. Pat. No. 4,847,172 (Maskalick et al.). Alternate sintering processes, not requiring expensive EVD equipment, offer a potential cost saving if they are able to meet all technical objectives. One of the serious problems encountered by simple sintering processes for forming the cermet fuel electrode has been microstructural changes in the cermet which, can cause deterioration of the SOFC voltage over time. Thus, what is needed is an improved electrode and method of electrode manufacture which meets all the previous requirements, but which uses materials that can allow ease of low cost fabrication.